Where a backrest of a chair-type massage apparatus is concerned, there is a heretofore known backrest having a built-in massage mechanism for operating a pair of right-hand and left-hand massage members in a manner to give a massage. This massage mechanism needs to be designed to move so as to conform to the spine of a user sitting on a seat of the massage apparatus, wherefore a vertical movement mechanism is provided therein for such a movement. The massage mechanism and the vertical movement mechanism constitute aback kneading device.
As a back kneading device of this type, there has hitherto been proposed a device having an advancing and retracting mechanism capable of causing a massage member to protrude forward beyond a normal massage position and causing it to return to the original normal position (refer to Patent Literature 1, for example). In this advancing and retracting mechanism, a support frame, which is moved up and down in a backrest by a vertical movement mechanism, is provided independently of a massage mechanism. The massage mechanism is supported on the support frame in such a manner that its upper-end side can be rocked back and forth about a lower pivot point, and in this state the mutually corresponding upper parts of the support frame and the massage mechanism can be moved in contact with and away from each other in a front-rear direction (back-and-forth driving operation).
There are four systems that exemplify the method to effect such a contact-separation operation: “Air system” using an air cell in which switching between an expansion mode and a contraction mode is done by admission and release of air; “Link system” in which, in a threaded shaft having a right-hand thread portion and a left-hand thread portion formed divisionally in its upper and lower parts in a vertical direction, and an internal thread body is threadedly engaged with each of the upper and lower thread portions, the upper and lower internal thread bodies being coupled to each other in the form of a toggle linkage by a pair of links; “Cam system” in which an eccentric cam is rotated while being fitted in a support body with a bearing; and “Lever system” in which a link (rocking lever) is rocked through an engagement between a worm gear and a threaded gear (pinion gear).